cleaner way to set pickler

Author: lgray

src/coffea/processor/executor.py

@@ -514,7 +514,7 @@ def __init__(
     ):
         if mp_context is None:
             mp_context = multiprocessing.get_context()
-            mp_context.reducer = cloudpickle.Pickler()
+            mp_context.reduction.ForkingPickler = cloudpickle.Pickler
         super().__init__(
             max_workers=max_workers,
             mp_context=mp_context,